The present invention relates to a hydraulic control apparatus of an automatic transmission and a method thereof, in particularly, to a hydraulic control for discharging air mixed into a hydraulic circuit.
Heretofore, there has been known an automatic transmission having a constitution in which engagement and release of friction engagement elements for gear change is controlled by a hydraulic pressure.
Further, there has been known a hydraulic control for periodically supplying a hydraulic pressure to friction engagement elements for gear change in released condition to discharge air mixed into a hydraulic circuit (refer to Japanese Unexamined Patent Publication No. 10-169764).
If, during the delivery of hydraulic pressure for air discharge, a gear change request is generated and a control is started to engage the friction engagement elements to which the hydraulic pressure has been periodically delivered for air discharge, the engagement control is started with an initial pressure higher than a normal initial pressure. Consequently, engagement timing is hastened, which may cause a gear change shock.
Therefore, it is desirable to terminate the air discharge control without delay when the air discharge is completed, so as to avoid as much as possible the generation of a gear change request during the air discharge control.
However, according to the conventional control, the hydraulic pressure is always delivered if a condition capable of supplying the hydraulic pressure is fulfilled, without determining whether or not the air discharge is completed, so it is highly possible that the gear change is started during the delivery of hydraulic pressure for air discharge.
Moreover, as in the convention technique, when a duty ratio in periodic hydraulic pressure delivery is changed according to operating conditions, it is difficult to judge precisely the completion of the air discharge based on the total delivery time.
Therefore, the present invention has an object to provide a hydraulic control apparatus and a hydraulic control method of an automatic transmission, capable of optimally controlling delivery of hydraulic pressure for air discharge, and also capable of judging completion of air discharge accurately.
In order to achieve the above object, the present invention is constituted so that a hydraulic pressure is periodically delivered to friction engagement elements in released states out of a plurality of friction engagement elements for gear change, to discharge air mixed into a hydraulic circuit, a duty ratio in the periodic delivery of hydraulic pressure is switched to a plurality of different values according to operating conditions, an achievement ratio of air discharge is calculated per each of the plurality of duty ratios, and when a sum of the achievement ratios reaches a predetermined value or above, the periodic delivery of hydraulic pressure for air discharge is terminated.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.